Can someone direct me to specs for the maximum weight that a tail-gate can support on a 2008 Tacoma?

The idea is to have two 350-Lb motorcycles on the bed, with the rear tire of each supported on the rear-most few inches of the tail-gate. But then, it must also simultaneously support the weight to two adults (say 200 Lbs each, assumed worst-case) standing on it while loading & unloading the bikes.